The invention relates in general to a suspension arrangement for a semiconductor accelerometer, and more specifically to such an arrangement which provides complete symmetry to the entire device.
Surface micromachining is an IC-compatible technology in which a thin film microstructure material (typically polysilicon) is deposited and patterned on a sacrificial spacer layer (typically silicon dioxide), and the spacer is subsequently removed to release the free-standing microstructures. This technology has been used by Analog Devices, Inc. to fabricate integrated accelerometers. The devices typically include a moving mass and fixed cantilever electrodes that are interdigitated within electrodes associated with the moving mass. When the structure experiences an acceleration in the sensitive axis the capacitance between the fixed and moving fingers is changed, and a measurable signal results. Suspension tethers are designed to resist motion in the non-sensitive in-plane axis.
Due to the very small scale dimensions of the elements of a semiconductor accelerometer, particularly the moving mass, several problems can occur. Since the materials used, e.g., polysilicon, result in a uniform stress gradient through the thickness of elements such as the mass, the elements tend to bow or warp. In addition the tensile stress of the mass tends to cause shrinkage. If portions of the mass are too low, thus closer to the substrate surface, there is the possibility of contact and sticking. If the mass contacts or sticks to the underlying substrate, the acceleration measurements become inaccurate. Also, the bowing or warping of elements can result in the electrode fingers not aligning well, thus affecting measurement performance. In all, such problems can be optimized by trimming the responses, however, such techniques are time consuming and expensive.